The backup size is about 335 MB, and the backup was taking a bit over 6 minutes.

I couldn't find a reference on where to change the CGI timeout.

YaBB 2.4 - Windows Server 2003/IIS

I am getting (at different times)

CGI TimeoutThe specified CGI application exceeded the allowed time for processing. The server has deleted the process.

and the classic:

application failed to return a set of headers.

I rebuilt the data in YaBB to make sure it wasn't choking on that - rebuilt with no problems. None of the users are getting errors on run-of-the-mill daily stuff. So I think its only the backup is a problem. I checked diskspace - 36 GB free

I'm going to wait on my next full server backup to implement the changes, since I have a bunch of ISAPI stuff running and don't wish to break my two very expensive (and poorly supported) toys. That 300 second limit is indeed the roadblock.